In Assassin's Creed Odyssey, the playable protagonist's maternal grandfather is Leonidas I.
Leonidas I is the Spartan king shown at the opening of the game during the Battle of Thermopylae. The playable protagonist, either Kassandra or Alexios, is descended from him through their mother, Myrrine. This family connection gives the hero a claim to Leonidas's broken spear and its Isu-enhanced abilities.
The spear is not simply a normal weapon. It is a damaged Piece of Eden that can be upgraded and is used to unlock abilities as the story progresses. The game connects this fictional technology to a historical setting and to the broader Assassin's Creed mythology.
Leonidas ruled Sparta from 489 to 480 BC, according to traditional historical dating. The real Battle of Thermopylae took place in 480 BC. Odyssey dramatizes the battle and then shifts into its main narrative decades later.